As I understood it, their point was that evolution does not add new layers, evolution of the brain always happens by modifying the existing structure.

Which is why a "stratified" view of the brain with evolutionary older layers near the center and newer layers near the surface is incorrect.

The implications of this belief then led to incorrect assumptions about both humans and (non-human) animals: That the human brain is an "animal brain plus something else" and therefore automatically superior - and inversely that animal brains are "human brains minus something" and therefore automatically inferior. The article argues against both positions.

The triune model doesn't claim our brain grows additively in rings like trees. It's merely an observation about the primary evolutionary origins of the three parts. Just those three.

And the superior/inferior characterization is not part of the triune model. You are free to interpret it that way if you want, but it's not part of it, so it's not a rebuttal.

Not sure about reptiles, but the author write this about mammals:

> Neurobiologists do not debate whether any cortical regions are evolutionarily newer in some mammals than others. To be clear, even the prefrontal cortex, a region associated with reason and action planning, is not a uniquely human structure. Although there is debate concerning the relative size of the prefrontal cortex in humans compared with nonhuman animals (Passingham & Smaers, 2014; Sherwood, Bauernfeind, Bianchi, Raghanti, & Hof, 2012; Teffer & Semendeferi, 2012), all mammals have a prefrontal cortex.

I also read some interesting papers a few years ago about corvids - particular New Caledonian Crows: Those animals do not have a neocortex and hence were thought incapable of many higher-level cognitive tasks, such as planning, tool use, etc. Turned out the crows were in fact capable of them. One hypothesis I read suggested that, as crow brains are structured differently, another structure may have taken the role that the neocortex has in humans.

So even if it's an anatomical distinction, it's an unreliable indicator of mental capacity.

Literally nobody is claiming that only humans have neocortexes.

Nor is anyone claiming it's an indicator of mental capacity. The recent discoveries about crows were interesting, but that doesn't have anything to do with the fact that the neocortex in mammals plays a particular, functional, well-recognized role.

> The triune brain consists of the reptilian complex (basal ganglia), the paleomammalian complex (limbic system), and the neomammalian complex (neocortex), viewed each as independently conscious, and as structures sequentially added to the forebrain in the course of evolution. According to the model, the basal ganglia are in charge of our primal instincts, the limbic system is in charge of our emotions and the neocortex is responsible for objective or rational thoughts.

We disproved:

- the sequentially added nature

- what these zones are responsible for

So in the end we're only left with a semi-arbitrary split of the brain in 3 zones and none of the hypothised implications. That to me refutes the hypothesis as a whole, and we're keeping the zone naming by sheer inertia.

The correct view is that while the neocortex is indeed mostly new, the “older” more central structures were modified throughout evolution.

This argument is akin to saying we're not "newer" apes ala

> The refutation is of the idea that it’s a strictly chronological ordering of species, with the old species still inside and intact. The correct view is that while homosapiens are indeed mostly "newer", the “older” apes were also modified throughout evolution

Obviously.

> As Paul MacLean (1964), originator of the triune-brain theory, stated,

>> man, it appears, has inherited essentially three brains. Frugal Nature in developing her paragon threw nothing away. The oldest of his brains is basically reptilian; the second has been inherited from lower mammals; and the third and newest brain is a late mammalian development which reaches a pinnacle in man and gives him his unique power of symbolic language.

And they quote other textbooks that are making claims along these lines too; this is right at the beginning of TFA. So I think you are wrong that “no one thinks that”.

Of course they don’t think the old brains are 100% static but there are claims that they are largely conserved.

That they're not static doesn't necessarily mean that they aren't largely conserved.

Modern reptiles, birds, and humans have experienced the same amount of evolution. There’s similar traits, but we don’t call whale flippers hands because that’s what the structures was in some of their ancestors or even what it is in related species. Similarly we don’t call human hands flippers because that’s what the structure started out as even further back and it’s still preforming that function in modern fish.

The human visual cortex is larger than most creatures entire brain, it’s a wildly different structure than you find on a frog which plays a wider role in cognition than just decoding vision.

Going back to your analogy saying visual cortex of frog is wildly different from a human visual cortex. That's similar to saying a frog backbone is wildly different from a human backbone. Sure that's true but there's also a shared common core functionality largely conserved across time

I think taking a macroscopic view like this points at interesting types of analysis. Evolution is analogous to a hill climbing algorithm navigating towards local optima in a probability field. Macroscopic evolutionary trends give a low resolution peek into the underlying probability distributions.

Frogs, birds, etc have very distinct spherical visual cortexes. Humans and squirrels don’t have that kind of spherical structure. So, other than naming where vision is processed the same thing the overlap isn’t particularly strong even at the most basic structural level.

It’s perfectly reasonable to extrapolate from studied organisms to species that share recent common ancestors. But reptiles don’t represent the probability distribution, they represent different results from that distribution under different evolutionary pressures. Extrapolation backwards and our common ancestor was very different from all decedents.

Every scientific model and analogy is flawed in some way, but many are still useful.

By noticing that while they have significant differences, those structures also have significant similarities, we were able to correctly answer one of the greatest questions of all time: where did humans come from?

Of all models, this one is particularly useful.

Compare the giant spheres used by snakes, bird, frog etc to process visual information and try and map that to a rodent, bat, or human. Hint you’re not going to be able to locate an obvious sphere because the fundamental morphology is just different. You can say we both have areas of the brain devoted to processing vision but so do fruit flies.

https://en.wikipedia.org/wiki/PAX6#Species_distribution

It would seem enough people did to warrant this person to write about the topic.

- The point of the criticism is not that the neocortex is not a "layer" at all, but that it is not the case that if you were to remove the neocortex layer, you would essentially get the brain of a lower-order animal--but this is what is implied by the triune theory.

It seems like this is what they are refuting. It is not so much a new layer as it is an evolution of existing structure.

https://en.wikipedia.org/wiki/Neocortex

It clearly states:

> The six-layer cortex appears to be a distinguishing feature of mammals; it has been found in the brains of all mammals, but not in any other animals.

What it evolved out of is entirely irrelevant -- everything evolves out of something else in some fashion.

The reptilian equivalent to the neocortex is the dorsal ventricular ridge which evolved separately and in parallel. This presents two problems to the hypothesis: first the much simpler DVR serves much of the same purpose as the neocortex which was completely unknown at the time and second the most interesting bird species (the smartest ones) often don’t have an equivalent structure at all. There isn’t even a clear relationship between intelligence, complexity, evolutionary age, etc. After 250 million years of evolution any similarities are accidents of random convergence.

As much as people joke about having a separate stomach for ice-cream, I've never heard anyone suggest that their "lizard stomach" would handle certain foods.

I am not qualified to argue for or against the Triune brain, but it seems easy to see why the brain is different from the stomach in this regard.

IMO the key is distinguishing between these two ideas:

1. There is a gross anatomical structure that can be linked to ancient ancestors with certain characteristics.

2. Those structures in living creatures are somehow "not really modern" or are unusually tied to the needs or limitations of those ancient ancestors.

Consider your fingers: They originate from fin-bones ~380 million years ago, yet (unlike "lizard brain") nobody talks about possessing "fish fingers" except as a fried food product. We also don't create narratives explaining our finger operations or design in terms of what ancient fish required or were capable of.

Humans have a neocortex, but if you play with a cat or dog, you can recognize and understand the emotions they are feeling. Fear, anger, joy, anxiety, relaxation, etc. That suggests the structures responsible for those things in us are probably not that different from them.

And yet birds don't have a neocortex, and yet they share all the same behaviors that mammals share (at least in some species). I don't know if there is some creature with a neocortex that doesn't share all of these behaviors, but at least we can see that the neocortex may be sufficient but is not required for a species to have them.

The idea that major organs are strictly conserved over 250 million years while something as fundamental as homeostasis drastically diverges is frankly a bit wackadoo.

Hmmm, my impression was that evolution took an existing structure and stuffed a bunch of new functionality into it, making it much larger and more complicated. Although the structure already existed in some form, it didn't have the same function. I think it's reasonable to think of this process as "adding" something, even if the structure itself is not entirely new. And human brains pretty clearly _are_ more intelligent than reptile brains!

Here's an analogy: Ancient organisms had light-sensitive "eyespots", and many of our distant cousins on the evolutionary tree (e.g. present-day flatworms) still have eyespots. Evolution gradually modified these existing structures into eyes. I think it's reasonable to think of eyes as a "new" organ that humans have and flatworms don't; and our eyes are clearly superior to flatworms' eyespots.

Point being its not about adding, it's about adapting

I agree there are other cases where evolution removes/simplifies things. Evolution is adaptation, and that adaptation can consist of adding, changing, and/or removing.

The human brain is a highly complex set of distinctive organs. Saying there are different types of brains forming the human nervous system is literally what the biology teaches us.

I imagine the reason Newton’s laws never developed before was because of all the influential know-it-alls who took empirical data (moving objects stop! Gaseous balloons rise!) and drew the wrong conclusions from it (gases want to rise up to be with all the other gases…solids want to be at rest.)

It took Newton to do a thought experiment of a projectile moving in outer space to deduce his laws of motion.

The fact is that newtons laws aren’t really observable on earth, unless you have the imagination to see it and do the mental bookkeeping of accounting for friction as a separate force.

The lizard, small mammal, human distinction maps pretty deceptively onto Aristotle's distinctions between the souls: vegetative (plant), sensitive (animal), and rational (human). So if one is trying to pinpoint the seat of intelligence, it seems to follow that we can ignore the two lower sections of the brain in favor of the higher one. Franz Joseph Gall, the founder of phrenology, himself did that, writing off the cerebellum as relevant only for producing the sexual drive [1].

Scientific theories of self-control which were nothing more than Christian dualist arguments evolved out of Gall's work and argued that intelligence involved suppression of the lower faculties, which provided cover for eugenicist and supremacist arguments throughout the 20th century and still shows up today in popular theories about how the 'limbic system' subverts the rational capacities of individuals and is used to manipulate the masses (Elon loves this theory).

Current work funded at the intersection of artificial intelligence and neuroscience still prioritizes the neocortex as the seat of rationality, with some like Jeff Hawkins (Palm founder turned brain scientist) arguing that "intelligence is an algorithm found in the neocortex". Singularity arguments rely in part on the assumption that intelligence in humans is mostly limited by the other parts of the brain, not empowered by them, and that a form of intelligence freed of embodiment will inevitably exterminate those that are embodied by right.

The truth is, neglected sub-regions such as the "lizard" cerebellum actually contain the vast majority of neurons, have been shown to have evolved disproportionately larger within early hominins [2], and are theorized to be equally involved in abstract cognition as in bodily manipulation [3]. This is something of a paradigm shift that has only been able to take shape since the late 20th-century (through the work of Jeremy Schmahmann, Peter Strick and others[4]), even though hints of it have been present in the data since it was collected, and that's because of how compelling the triune brain model has been. Research in this direction can directly address mental illnesses such as schizophrenia, but it has to be funded first [5].

[1] https://www.frontiersin.org/articles/10.3389/fnana.2019.0004...

[2] https://pubmed.ncbi.nlm.nih.gov/25283776/

[3] https://www.sciencedirect.com/science/article/abs/pii/S03043...

[4] https://www.sciencedirect.com/science/article/pii/S089662731...

[5] https://www.youtube.com/watch?v=5UUqKuhvTk0

In general I think this effect contributes to a lot of "over-debunking". We see way over-simplified, yet very loosely accurate, mid 20th century scientific models like the triune brain, "ontogeny recapitulates phylogeny", the left-brain vs. right-brain, and the idea that differences in language contribute to differences in cognition; and then silly people take these models way too far and use them to justify dubious things; and then they become over-debunked to the point that speaking them aloud immediately ostracizes you as some outdated bigot; while the whole time the models themselves have been reasonably OK high-level starting points for discussion that obviously need revision for any lower-level details.

> The truth is, neglected sub-regions such as the "lizard" cerebellum actually contain the vast majority of neurons, have been shown to have evolved disproportionately larger within early hominins [2], and are theorized to be equally involved in abstract cognition as in bodily manipulation.

The relative number of neurons is not evidence for or against the model, nor the fact that they were larger in early hominins. Showing their involvement in abstract cognition is more interesting, but that's only evidence against the triune brain if you make the exact same mistake that you're criticizing, which is assuming that "abstract cognition" is some high-level uniquely human (or primate) trait. If that exact "abstract cognition" also exists in reptiles and birds (and it appears to), then the fact that the cerebellum contributes to that cognition is not evidence against Triune Brain.

I think its important for people to also spread the idea that "this is known to not be correct". Bad mental models continue to proliferate for generations (bad here not being a value judgement but rather of their known incorrectness and lack of predictive power).

It's like when you tell people that the alpha/beta dynamic amongst wolves came from one flawed study that has not been replicated and shown to be false many many years ago now. Same with "learning styles" research. Its difficult to approach the subject in a way that doesn't cause people to get defensive sometimes - they like the simplistic model which may once have been a fine point for a beginner but they got stuck in it and it can impair their growth and understanding.

It's fine for a layperson to walk around believing whatever they like - its likely closer to the truth than whatever they might otherwise have thought, or at least got them thinking. It becomes a problem when people actually base decisions off these things.

Is the simple metaphor of 3 layers in the brain equivalent to saying gasses want to be together and solids want to rest?

I think part of the debate in the comment section is on what kind of order / pattern we are trying to capture with the analogy. Does it make more sense to be analogous to the structural observations, or a more functional equivalency?

You could say an ocean is like a desert in that they are vast and empty with respect to surface structures observed by a human traveller, but obviously from a functional /environmental perspective the two almost couldn't be more dissimilar

Based on the rather casual way the author is using language in this piece, I'd bet that the researchers forgot that what they are describing are (nested) model(s) of reality...or that that level of precision is "pedantic" (the consequence being the confusion in this comment section).

And do living reptiles, today, not also "have a basal ganglia which evolved from reptiles?" seems we've a name collision, here. Perhaps we should refer to our ancestors as "proto-reptiles," or else our contemporaneous cousins as "post-reptiles" whose brains have had just as many years' time to depart from our common reptilique ancestor.

Mammals and reptiles share a common ancestor with an ancient amniote, not a reptile

> Synapsids[a] are one of the two major clades of vertebrate animals that evolved from basal amniotes, the other being the sauropsids, which include reptiles (turtles, crocodilians and lepidosaurs) and birds.

> the only extant group that survived into the Cenozoic are the mammals.

> The animals (basal amniotes) from which non-mammalian synapsids evolved were traditionally called "reptiles".

> It is now known that all extant animals traditionally called "reptiles" are more closely related to each other than to synapsids, so the word "reptile" has been re-defined to mean only members of Sauropsida (bird-line Amniota) or even just an under-clade thereof

Yes. Anything this simple will be wrong. Almost everything you learn in school before graduate level courses will be wrong. Most of it won't matter to you unless you start working in that field.

[1]https://en.wikipedia.org/wiki/Reality_Is_Not_What_It_Seems

From what I understand, in many fields its pretty clear that most of the stuff in graduate courses is, too, its just that then next step toward right is less clear and more disputed than for the earlier wrong stuff.

If the triune brain model was completely false (as stated by the caricature of "brain is not an onion with a tiny reptile"), it would not be straightforward to even identify the neocortex -- how do you know it is 'neo', what is it a 'neo' of and so on?

So the fact that we can meaningfully talk about these areas of the brain suggests that there is in fact continuity and building-upon happening in our evolutionary journey, although of course it's not like the cartoon they show (which I have never seen before from anyone seriously talking about this topic, I might add).

This is a sort of proof-by-etymology and it isn't very persuasive. We have all kinds of words that derive from misunderstandings that have stuck, even long after the original misunderstanding was resolved. The geographical equivalent of this would be to say "we call them Indians, so they must have originated from India at some point!"

It's entirely possible that as our understanding of the brain evolves, this layered-over-time model will be seen as a quaint misunderstanding, but we'll still use the original names for the sub-organs because there's too much inertia to change them.

I don't think that the intention of the article is to say it's completely false. It's that there is no neat evolutionary separation between the different parts of the brain. The "reptile" brain in humans is not truly the brain of a tiny reptile. We inherited a similar structure, but it evolved with us just like the rest of our brain.

The same goes for the tiny reptiles we see today, their brains also evolved alongside them. If we truly had a tiny reptile inside our brain then logically we could simply study the behavior of tiny reptiles if we wanted to study that part of the human brain. Which is probably the kind of misconception the authors of the article want to refute.

I took this as the key point addressed in the introduction—this model of evolution is widely understood to be wrong. We did not evolve from reptiles, we evolved alongside reptiles from a common ancestor. Modern reptile brains have also changed in form and function since our common ancestor, and we don't have whole divisions of the brain that exist in humans but not in reptiles.

Essentially, the triune brain theory goes right alongside that classic image of the monkey evolving into a man. It's not actually how evolution works, but it's a catchy simplification that's hard to get rid of.

From the caption to the first diagram:

> The evolutionary tree (c) illustrates the correct view that animals do not linearly increase in complexity but evolve from common ancestors. The corresponding view of brain evolution (d) illustrates that all vertebrates possess the same basic brain regions, here divided into the forebrain, midbrain, and hindbrain. Coloring is arbitrary but illustrates that the same brain regions evolve in form; large divisions have not been added over the course of vertibrate evolution.

https://archive.org/details/temporallobelimb0000gloo/mode/2u...

Your "isn't it true" paragraph is all incorrect (unless it is considered in a uselessly general way). For instance, Gloor notes that all mammals have an isocortex (that seems to have started in olfactory cortex). The "reptiles" that humans evolved from is just a catch all terminology for a diverse group of life and not closely related to current reptiles. He starts the chapter noting that ancestral brains are not available for direct study.

> The triune brain hypothesis doesn't seem "wrong", it just seems like a simple categorization that is useful for making big-picture distinctions.

Right, but having worked in the field, I can assure you that there are, in fact, too many practicing psychologists, cognitive scientists, and even neuroscientists who believe the triune model of the brain to be literally and, sometimes, absolutely true. These are the types of people whom the linked paper is trying to reach.

Misunderstandings based on these oversimplified models are driving the current debate around modular versus distributed computation in the brain[1]. Obviously, a more accurate model of the brain would account for both ideas, but there is growing concern in the neuroscientific community over the amount of grant money going toward defending older, dead-end modular models instead of improving newer, more promising distributed models, mostly as a result of entrenched interests prioritizing the maintenance of prestige over the pursuit of truth.

In short, putting bad models on blast is good and necessary for the advancement of science. You can get a lot done with the plum pudding model[2] of the atom, but you can get far more done with the Bohr model[3] which emerged only after Rutherford, Bohr, and several other physicists published several iterative takedowns of the former, and yes, they too had to deal with entrenched interests who operated under the assumption that the plum pudding model was literally and absolutely true. It took a decade of experiments and several increasingly correct models before academic consensus shifted enough to accept the existence of subatomic particles and academic consensus began its collective investment in quantum mechanics. We're now in a similar place with neuroscience in the tension between modular computational models, which includes the triune brain model, and distributed computational models, which are showing promise in rescuing fMRI studies with their strong modular tradition from the replication crisis[4].

[1] https://www.cell.com/trends/cognitive-sciences/fulltext/S136...

[2] https://en.wikipedia.org/wiki/Plum_pudding_model

[3] https://en.wikipedia.org/wiki/Bohr_model

[4] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5457304/

(The rest of the article seems much more defensible to me: well-written without quite verging into strawman territory. But I'm no expert in the field.)

The title hints at what's coming. I mean of course no one thinks there's a "tiny reptile" inside our brains, ha ha.